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Green Party co-leader Russel Norman has made a sharp personal attack on Prime Minister John Key, all but shutting the door on working with National after the next election.

Key described Labour and the Greens as the "devil-beast"; Norman hit back at his party's annual conference yesterday by labelling Key "corrosive" and "extremely divisive", repeatedly comparing him to former Prime Minister Robert Muldoon.

"Next time you see John Key smiling, remember he's not smiling because he likes you, he's smiling because he's giving favours to his mates while undermining your democracy," he told an audience of around 120 people in Christchurch.

The Greens have previously avoided personality politics. Asked whether his speech signalled a new approach, Norman said: "It's important to put a line in the sand about what's happening to our constitution and our democracy."

After the speech, he would not rule out working with National if it gained a third term - but his tone of contempt for the National-led Government's "attacks on democracy" and "dodgy deals" made it clear that this would be unlikely.